Corrugated wave guide



June 21, 1955 KRUTTER r 2,711,517

CORRUGATED WAVE GUIDE Filed Sept. 14, 1945 VIJIIIIIII INVENT S HARRY KRUTT HERBERT GOLDSTEIN ATTORNEY CORRUGATED WAVE GUIDE Harry Knitter, Brookline, and Herbert Goldstein, Cambridge, Mass, assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application September 14, 1945, Serial No. 616,416

8 Claims. (Cl. 333-?) This invention relates in general to the transmission of high frequency electrical waves and more particularly concerns a novel wave guide structure.

It is often desirable to modify the wave length of electrical oscillation of a given frequency in a wave guide or other transmission line systems. As an example, in the design of antennas consisting of linear arrays for providing a beam having a particular radiation pattern, it is necessary to obtain the proper phase relations between the elements of the array. For beam radiation it is necessary to shorten the Wave length, and for this purpose it is most convenient to use a wave guide or similar transmission line wherein the wave length is less than the free space wave length.

It is well known in the art that shortening of the wave length within a wave guide transmission system may be readily accomplished by the substitution of a solid dielectric material for the more conventional air insulation. The use of a solid dielectric material in a transmission line results in the introduction of electrical and mechanical problems not ordinarily encountered in air or gas filled lines. The accumulation of moisture and foreign substances for example involve an electrical breakdown hazard, particularly serious in dielectric filled lines in that breakdown results in carbonization, or permanent line failure.

A transmission line possessing the property of shortening the wave length therein is known as the corrugated transmission line and our present invention contemplates 21 specific adaptation of the principles of this line to a wave guide line structure.

It is therefore an object of our present invention to provide a transmission line system for high frequency electrical energy wherein the wave length is a predetermined amount less than the corresponding oscillation wave length in free space.

Another object of our present invention is to provide a novel corrugated wave guide structure which provides considerable shortening of the electrical wave length of energy transmitted therethrough.

A further object of our invention is to provide 2. rectangular wave guide for the transmission of high frequency energy wherein a plurality of projections of conductive material are integrally attached to the central portion of the internal surface of one of the wave guide walls.

These and other objects of our present invention will now become apparent from the following detailed specification taken in connection with the accompanying drawings in which:

Fig. l is a general perspective view of the corrugated wave guide contemplated by our invention;

Fig. 2 is a cross sectional view taken in a plane through the longitudinal axis of the wave guide of Fig. 1 and parallel to one of the broad walls thereof; and

Fig. 3 is a cross sectional view of the novel wave guide structure taken in a plane through the longitudinal axis of the guide and parallel to one of the narrow Walls thereof.

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Patented June 21, i9 5 As hereinabove stated, the general problem of which this invention is a specific solution is that of the theory of corrugated transmission lines. Considerable art exists related to the rigorous solution of the effects of conductive corrugations upon the wave length in transmission systems and the experimental verification thereof. As an example, reference is made to copending application of Milton G. White, Serial No. 504,777, filed October 2, 1943, and entitled Control of Wave Length in Wave Guide, now U. S. Patent No. 2,567,748. issued September 11, 1951. A generalized non-analytical discussion of the corrugated transmission line problem follows herein.

The simplest type of corrugated line is the parallel plate type in which one conductor comprises a plane plate and the other comprises a parallel plate which is corrugated, that is, has substantially rectangular slots cut therein and extending transversely to the direction of energy propagation in the parallel plate line. The line extends indefinitely in all directions. If the width of a slot is small compared to a wave length, then the electric field at the mouth of slot must be almost entirely in the direction of propagation. Ordinarily, however, in a parallel plate line, the principal mode of propagation has no component of electric field or magnetic field in the direction parallel to the longitudinal axis, that is, the axis of propagation. In order to satisfy the electric field boundary conditions in the corrugated parallel plate line and create an electric field component across a slot in the direction of propagation, the wave length in the line, that is AZ must be smaller than the free space wave length h. A rigorous analysis of the corrugated parallel plate line has indicated a solution for the ratio of x /x as given by the following equation:

dL mz In this equation of the slotted plate.

This equation is indicative of a shortening of the wave length within the transmission line and further indicates that the effective decrease in wave length is dependent only upon the geometrical configuration of the line. Frequency is not a factor so long as the free space wave length is considerably greater than the spacing between consecutive slots. The effective shortening of the wave length is greatest as the line is operated in the region of resonance, that is, When the wave length is equal to 4 times the slot depth, or 4L.

Shortening of the wave length in transmission may be accomplished by carrying the basic principles of the parallel plate corrugated transmission line over into other conventional lines, as for example, the coaxial and wave guide lines. The behavior of corrugated wave guides and coaxial lines is essentially the same with the exception that the shortening of the Wave length cannot be as great as obtained with the parallel plate line. On the other hand, the physical structure of the parallel plate line precludes its use in application Where considerable shortening of the wave length is required.

In Figs. 1, 2, and 3 there is shown a wave guide construction wherein the shortening of the Wave length very closely approximates the shortening obtained in the parallel plate corrugated transmssion line. The wave guide 11 is of rectangular cross section and comprises two parallel broad walls 12 and 13 and two narrow walls 14 and 15. The bottom broad wall 13 of the wave guide 11 is provided with a plurality of integral or otherwise secured metallic block-like projections 16 separated by correspondingly rectangular slots 17 which are normal to the longitudinal axis of the wave guide 11.

As clearly illustrated in the perspective view Fig. 1 and the accompanying cross sectional views Figs. 2 and 3, the projections 16 do not extend across the entire surface of the broad wave guide wall 13, but are centrally disposed thereon and provide a pair of channels or end zones 21 and 22. The projections 16 extending into the wave guide interior are preferably uniformly spaced along the longitudinal axis thereof, and are proportioned so that there are a plurality of such projections for each wave length in the guide. it may be shown that if there are less than two such projections 16 per wave length, a cut off point is reached where no energy may be successfully transferred through the wave guide.

When the width of the end zones 21 and 22, that is, when the dimension A minus C is large as compared to the dimension 2A, the shortening of the wave length is approximately that given for the corrugated plate transmission line. Therefore, the formula given above may be used with a fair degree of accuracy for the corrugated wave guide transmission line having large end zones. The dimensions for substitution in the formula are marked upon the Figs. 2 and 3.

The correspondence of the formulas for shortening of wave length in the parallel plate line and the corrugated wave guide with end zones is most correct if the wave guide is operated in the region of resonance, that is, when the wave length is equal to 4L, or, four times the depth of a slot or space between projections 16 in the wave guide. As the size of the end zones 21 and 22 is decreased, the wave length within the guide correspondingly increases. When AC=0, the end zones 21 and 22 disappear and a corrugated wave guide is obtained wherein the projections extend across the entire surface of one of the broad walls. Although the wave length in the guide for this condition is less than the free space wave length, the difference between the two is small.

A rigorous treatment of the corrugated wave guide with end zones involves the use of field theory and will not be presented here. However, a physical explanation for the correspondence between the equation for a parallel plate corrugated line and corrugated wave guide with large end zones will now be discussed.

One of the boundary conditions which must be satistied in a metallic conductive wave guide is that the magnetic lines of force must at all times be parallel to the surface of contacting conductors, that is, the normal component of the magnetic field at the surface of a conductor must be equal to zero. in the corrugated parallel plate line, the magnetic lines of force in each slot are normal to the direction of energy propagation. Inasmuch as the plates are infinite in extent,'the lines of force in a slot are undisturbed, straight lines joining with the flux lines lying in another slot only at infinity. In a corrugated wave guide withoutend zones, the linkage between flux lines in different slots results in a distortion of the magnetic field which is best described as leakage from a slot. The addition of end zones provides a pair of channels, such as 21 and 22 in Fig. 2, wherein the linkage of magnetic field lines may occur without distorting these lines from their normal plane which is parallel to the broad walls 12 and 13 of the wave guide. These end zones, therefore, serve as the infinity in the parallel plate case.

The attenuation in a corrugated wave guide having end zones may be calculated from basic field concepts. The resulting formulas are complex, but indicate qualitatively that the attenuation is least when large end zones are used and that the attenuation increases rapidly as the size of the end zones is reduced. Physically the explanation for this fact is that as the size of the end zones 21 and 22 is reduced there results an intense magnetic field in the narrow end zones, generating a comparatively high current density in the adjacent wave guide walls with correspondingly high attenuation. Hence the design of the corrugated wave guide must be guided by the various relations governing the shortening of the wave length and the attenuation as related to the size of the end zones and the projections into the wave guide.

Various other methods of applying the principles of the present invention for the shortening of Wave length within transmission lines may now be apparent to those skilled in the art. We prefer therefore not to be limited by the specific disclosures hereinabove set forth, but by the scope of the appended claims.

We claim:

1. A hollow wave guide for the transmission of high frequency electrical energy, said wave guide having a plurality of projections on a single inner surface thereof extending in the direction of energy propagation, the dimension of said projections transverse to the direction of energy propagation in said wave guide being less than the dimension of said surface transverse to said propagation direction.

2. A hollow wave guide for the transmission of high frequency electrical energy wherein the wave length of said high frequency energy within said wave guide measured in the direction of propagation is less than the free space Wave length of said high frequency energy, said wave guide having a plurality of uniformly spaced projections on a single inner surface thereof, said projections occupying the central portion of said surface and extending in the direction of energy propagation.

3. A substantially rectangular metallic wave guide for the transmission of high frequency electrical waves having two broad walls and two narrow walls, said wave guide having on a single broad wall a series of longitudinally spaced metallic projections occupying the central portion of said wall and extending into the interior of said guide, said projections being of substantially rectangular cross-section and having the dimension transverse to the direction of energy propagation in said wave guide substantially less than the corresponding dimensions of said broad wall, whereby, the wave length of energy flowing within said wave guide is a predetermined amount less than the wave length of said energy in free space.

4. A substantially rectangular metallic wave guide for the transmission of high frequency electrical waves having two broad walls and two narrow walls, said wave guide having on a single broad wall a series of longitudinally spaced metallic projections occupying the central portion of said wall and extending into the interior of said guide, said projections being of substantially rectangular crosssection and having the dimension transverse to the direction of energy propagation in said wave guide substantially less than the corresponding dimension of said broad wall, thereby providing a pair of comparatively large unobstructed end zones between said narrow walls and said projections, whereby the wave length of energy flowing within said wave guide is a predetermined amount less than the wave length of said energy in free space.

5. A substantially rectangular hollow wave guide for the transmission of high frequency electrical energy, said wave guide having two broad and two narrow walls, and having on a single broad wall a series of longitudinally spaced metallic projections extending into the interior of said guide, said projections being spaced longitudinally of said guide and having the dimension transverse to the direction of energy propagation in said wave guide substantially less than the corresponding dimension of said broad wall.

6. A substantially rectangular hollow wave guide for shortening the wave length of high frequency energy being propagated therein comprising, a series of longitudinally spaced, rectangular, metallic projections extending into the interior of said guide from one of the walls thereof, the long sides of said rectangular projections being normal to the longitudinal axis of said guide, said projections being centrally disposed transversely of said guide and formed in accordance with the equation wherein A is the wave length of energy within said guide, A is the free space wave length of said energy, d is the spacing between adjacent surfaces of successive projections, D is the spacing between corresponding surfaces of successive projections, L is the height of said projections, and X is the distance from the plane of the extremities of said projections to the opposite wall of said guide.

7. Wave guide apparatus for shortening the wave length of high frequency energy propagated therein comprising, a rectangular metallic wave guide having two broad walls and two narrow walls, one of said broad walls being formed to provide a series of longitudinally spaced metallic projections extending into the interior of said guide, said projections being of substantially rectangular cross section and having a dimension transverse to the direction of energy propagation in said wave guide substantially less than the corresponding dimension of said broad wall, said projections being located symmetrically with the longitudinal axis of said wave guide thereby providing a pair of comparatively large unobstructed end zones between said narrow walls and said projections, said projections further having a dimension in the direction of energy propagation in said wave guide to provide a plurality of projections per wave length of the energy propagated in said wave guide. 1

3. Wave guide apparatus for shortening the wave length of high frequency energy propagated therein comprising, a rectangular metallic wave guide having two broad walls and two narrow walls, the central portion of a single broad wall being corrugated to define a series of spaced projections extending in the direction of energy propagation and a pair of comparatively large unobstructed end zones between said projections and said narrow walls.

References Cited in the file of this patent UNITED STATES PATENTS 2,338,441 Kohl Jan. 4, 1944 2,374,498 Quayle Apr. 24, 1945 2,395,560 Llewellyn Feb. 26, 1946 2,396,044 Fox Mar. 5, 1946 2,403,252 Wheeler- July 2, 1946 2,409,227 Shockley Oct. 15, 1946 2,477,510 Chu July 26, 1949 2,544,842 Lawson Mar. 13, 1951 2,567,748 White Sept. 11, 1951 

